1. Field of the Invention
The present invention relates to a radial friction bearing, particularly for rotating rolls of a paper or textile processing machine, comprising a central shaft member and an annular sleeve member mounted coaxially with reference to the shaft member, whereby a gap is formed between said shaft member and said sleeve member to receive a lubricant, and whereby either said sleeve member is rotatably mounted on the stationary shaft member or said sleeve member is stationary and rotatably receives said shaft member.
The radial friction bearing assembly of the present invention is particularly suitable in connection with the rotatable mounting of essentially cylindrical conveying rolls and processing rolls used in the paper and textile industry. In connection with the support of such rolls two basic problems exist.
Firstly it is of paramount importance that such rolls be in a position to rotate, under certain circumstances during very extended periods, with very low rotational speeds, i.e. with a relative tangential speed between stationary and rotatable part of about 0.5 to 1 meters/sec. Simultaneously it must be warranted that a reliable operation at the normal operating speed, which may be quite high, is possible. In any case it must be ensured that an uninterrupted lubrication film be present, that means, that the stationary and the rotatable parts never come into contact one with the other one. The hydrodynamic forces of the lubricant, therefore, have to develop a sufficient supporting power even at very low speeds to provide under any circumstances an uninterrupted lubrication film between the stationary and the rotating parts of the bearing assembly. The problems occuring at very low rotational speeds could be solved by using a very highly viscous lubrication fluid. However it must be considered that not only the hydrodynamic power increases with increasing rotational speed, but also the energy consumption; the latter one will proportionally increase with reference to the rotational speed, on the one hand, and with reference to the viscosity of the lubricant, on the other hand. If one uses a normal lubrication oil, the situation will occur that the energy consumption of the bearing assembly for e.g. a paper roll rotating with a considerably high speed will be such that an economically tolerable limit is surpassed. Due to these considerations a radial friction bearing assembly should be constructed, as far as its hydrodynamical behaviour is concerned, in a way that enough hydrodynamic power is developped to provide a trouble-free operation, even if a very low viscous lubricant is used and even if the roll is rotated at very low speed. In the case of paper processing rolls, water has prooved to be a very suitable, low viscous lubricant.
Secondly an important problem in supporting such rolls may be seen in the fact that vibrations may occur, even if the rotating parts of the bearing assembly are dynamically balanced. Such vibrations may be caused by an insufficient hydrodynamic system of the bearing assembly, i.e. by a lubrification film which is not in a position to provide a correct distribution of the required hydrodynamic power at all rotational speeds. A roll supported by a hydrodynamic friction bearing assembly is required to operate also at critical speeds without the occurence of undesired vibrations.
2. Prior Art
Radial friction bearing assemblies of a general kind are well known in the prior art in different embodiments. However, depending of the fact whether the load in radial direction is very high, as e.g. in the case of axle bearings of railroad vehicles, or the load is comparatively low. e.g. in the case of rotating conveying rolls, different problems may occur; numerous disclosures refer to a solution of these specific problems.
Axial friction bearing are known, i.e. friction bearing which have to take a load effective in the direction of the axis of the bearing assembly, which comprise pockets adapted to receive a lubricant and being arranged on the flat surface of the stationary member of the bearing assembly (C. F. Kettleborogh, "An Electrolytic Tank Investigation into Stepped Thrust Bearings", Proceedings of the Institution of Mechanical Engineers, 169, (1955), pp. 679-688). However the problems in connection with axial bearing assemblies are quite different from the problems in connection with radial bearing assemblies, and solutions which might be well suited for one kind of bearing assemblies cannot be simply transformed to the other kind of bearing assemblies, or if even possible, only with great difficulties. This is particularly true in the case of prevention from undesired vibration which normally occurs only at radial friction bearing assemblies.
The U.S. Pat. No. 4,120,544 discloses a radial friction bearing comprising a stationary sleeve member, a rotatable shaft member mounted therein and a supply of lubricant to be fed between the stationary and the rotating part of the bearing. The bearing surface of the rotating shaft member is provided with grooves and is further subdivided in the circumferential direction into several partial surfaces, each of which has a flute or groove running in axial direction and other pumping grooves which build up pressure and which are on both sides of this flute opposite to one another and inclined at an angle to the circumferential direction. The individual partial surfaces are separated from each other by choke surfaces which limit flow of the lubricant.
It seems to be an object of the invention of the U.S. Pat. No. 4,120,544 to provide, with reference to known bearing assemblies of this kind, an improved load receiving capacity in both directions of rotation, simultaneously keeping in mind to ensure an economical manufacturing of such a bearing assembly. According to this Patent these objects should be met by the provision of a bearing assembly in which the shaft member comprises, on its outer surface, a plurality of groups of grooves and being divided, in circumferential direction, into several partial surfaces, and further by the fact that each partial surface comprises an axially extending groove fed with a lubricant and including a plurality of self-pressurizing grooves on both sides of this central axial groove, said self-pressurizing grooves extending from said central groove in an inclined, circumferential direction.
However it is believed that such an axial friction bearing assembly is not suitable at all for an application where a rotatable bearing member moves but with a very low speed with reference to a stationary bearing member, since no higly loadable lubrification film may be built up between the rotatable and the stationary part at low speed, because the overall cross section of the lubrication grooves is too high. Furthermore no measures can be seen in the disclosure of the above mentioned Patent to avoid the above cited higly undesirable vibrations.